Recent developments in the fabrication of liquid crystal display devices have been directed toward materials which display images by exploiting the light scattering properties of liquid crystals entrapped in discrete quantities in a plastic matrix. The display characteristics of these materials depend on the size and morphology of the entrapped liquid crystals. Characteristics such as scattering efficiency and switching time before ON and OFF states are affected by the diameter and density of the discrete quantities of liquid crystal.
Materials useful for light scattering displays occur when the diameter of the entrapped liquid crystals is such that they scatter incident light, i.e., on the order of about 0.2 to 10 microns in diameter.
Attempts at controlling the volume in which liquid crystals are entrapped have included imbibing liquid crystals into microporous plastic sheets, as disclosed in U.S. Pat. No. 4,048,358 and emulsifying liquid crystals with an immiscible binder such as polyvinyl alcohol to form encapsulated liquid crystal droplets, as disclosed in French Pat. No. 2,139,537 and U.S. Pat. No. 4,435,047.
These prior art techniques involving the mechanical entrapment of liquid crystals have some drawbacks. Encapsulation by emulsification can yield a relatively broad spectrum of capsule diameters which can necessitate sizing or sieving to achieve uniformity. Entrapment by imbibing into microporous plastic may obviate the sizing problem, but presents the further problem of sealing the micropores to prevent leakage of the liquid crystals.
Co-pending U.S. patent application Ser. No. 776,831, the disclosure of which is incorporated by reference, describes a light modulating material comprising droplets of liquid crystal dispersed in a solid, cured polymer prepared by dissolving the liquid crystal in uncured polymer and thereafter curing the mixture to cause the liquid crystal to phase separate as microdroplets.
Co-pending U.S. patent application Ser. No. 866,216, the disclosure of which is incorporated by reference, teaches the fabrication of light modulating material comprising microdroplets of liquid crystal in a thermoplastic matrix prepared by dissolving the liquid crystal in thermoplastic polymer to form a homogeneous solution and thereafter phase separating the solution to create in situ microdroplets.
Co-pending U.S. patent application Ser. No. 879,327, the disclosure of which is incorporated in reference, describes the preparation of a bistable memory material with high resistivity and permittivity comprising at least 50% by weight liquid crystal prepared by the phase separation of a homogeneous solution of liquid crystal and polymer.
As distinguished from prior art describing mechanical entrapment of liquid crystals, U.S. patent application Ser. Nos. 776,831, 866,216, and 879,327 describe materials having the advantages of ease of preparation and control of microdroplet size.